How many amps does the fan/heater require? If it's 20A then you need 12/3.

You should not have any 14 gauge wire on that circuit if it is connected to a 20 amp breaker.
If you can't replace the wiring, the breaker needs to be changed to a 15 amp.
If this is the only circuit run to the bathroom, you have a problem because code requires a 20 amp circuit.
If the twenty amp circuit also feeds other bathrooms, you have another problem.
Tell us a little more about what is going on.